I deleted out all unnecessary leaders for my mod and the game loads up just fine. When I try to actually play it won't begin and besieges me with a bunch of diplomacy info errors that never end (they just cycle through the names). Do I need to go in the diplomacy infos and start deleting? Not sure what to do.

Yes, you need to go into the diplomacyinfos.xml and delete the leaders code for the leaders you removed.

For some reason, the game loads the Diplomacy file at game startup, not during the initial boot.

Anytime you delete a tag, you have to find each file they're referenced in and delete them.

it does the same thing with the events and eventtriggers, so if you remove for example some techs, units, or civics you'll have the same problem, only I think it crashes (as opposed to only xml errors) when the event stuff is bad.

The reason it doesn't load till you try to start a game is that it doesn't display in the Civilopedia or game start screens. Anything which isn't NEEDED immediately isn't loaded until after you try to start a game running to cut down on the load times (well, technically just to break up the load times into 2 phases)